I believe the 500n uses EF mount lenses, which will work on the 50D, albeit there is the crop factor that Ryan mentions. This means that, for instance, your 50mm lens will become effectively an 80mm lens. This is not inherently bad; Canon's L lenses are all full frame (aka FX) lenses, and they are excellent and highly recommended for use with the 50D.
If you want one walkaround lens that will serve most purposes, the EF-S 17-55 f2.8 IS USM is generally considered to be very good (though not as well-built as the L lenses). Ryan, you neglected to mention that the Nikon lenses cost significantly more than the Canon lenses where there is an equivalent lens. There are plenty of people who will support your assertion that if cost is no object, Nikkor lenses are better, but for most people the cost could well be a significant consideration.
It can also be argued that in the crop factor (DX) range, Canon's offerings are much more diverse and better than Nikon's, and it's not until you start comparing the FF (FX) lenses that Nikkor has the upper hand.
I'm not entirely sure what you mean when you refer to the ergonomics, but if you mean the comfort of holding the camera in your hands and operating the controls, this is something that to a large degree is subjective. (I have not compared the two cameras you mention, but when I compared a Nikon D60 to a Canon 450D, I also preferred the feel of the Nikon; not everyone will agree).
The noise issue is one that has been mentioned often with the 50D. The high resolution combined with the sensor size makes it even more important to have good lenses, preferably fast ones so you don't have to use the high ISO so much. I strongly recommend getting very good lenses if you get the 50D. If you know what focal lengths you are most interested in, I'll make some specific recommendations. And of course dpreview.com has pages and pages of (sometimes strongly expressed) opinions on this subject.